The law of war is considered distinct from other bodies of law—such as the domestic law of a particular belligerent to a conflict—that may provide additional legal limits to the conduct or justification of war.

Attempts to define and regulate the conduct of individuals, nations, and other agents in war and to mitigate the worst effects of war have a long history. The earliest known instances are found in the Mahabharata and the Hebrew Bible (Old Testament).

In the Indian subcontinent, the Mahabharata describes a discussion between ruling brothers concerning what constitutes acceptable behavior on a battlefield:

One should not attack chariots with cavalry; chariot warriors should attack chariots. One should not assail someone in distress, neither to scare him nor to defeat him ... War should be waged for the sake of conquest; one should not be enraged toward an enemy who is not trying to kill him.

An example from the Deuteronomy 20:19–20 limits the amount of acceptable collateral and environmental damage:

When thou shalt besiege a city a long time, in making war against it to take it, thou shalt not destroy the trees thereof by forcing an axe against them: for thou mayest eat of them, and thou shalt not cut them down (for the tree of the field is man's life) to employ them in the siege: Only the trees which thou knowest that they be not trees for meat, thou shalt destroy and cut them down; and thou shalt build bulwarks against the city that maketh war with thee, until it be subdued.[2]

Also, Deuteronomy 20:10–12, requires the Israelites to make an offer of peace to the opposing party before laying siege to their city.

When you march up to attack a city, make its people an offer of peace. If they accept and open their gates, all the people in it shall be subject to forced labor and shall work for you. If they refuse to make peace and they engage you in battle, lay siege to that city.[3]

Similarly, Deuteronomy 21:10–14 requires that female captives who were forced to marry the victors of a war could not be sold as slaves.[4]

In the early 7th century, the first Caliph, Abu Bakr, whilst instructing his Muslim army, laid down the following rules concerning warfare:

Stop, O people, that I may give you ten rules for your guidance in the battlefield. Do not commit treachery or deviate from the right path. You must not mutilate dead bodies. Neither kill a child, nor a woman, nor an aged man. Bring no harm to the trees, nor burn them with fire, especially those which are fruitful. Slay not any of the enemy's flock, save for your food. You are likely to pass by people who have devoted their lives to monastic services; leave them alone.[5][6]

Furthermore, SuraAl-Baqara2:190-193 of the Koran requires that in combat Muslims are only allowed to strike back in self-defence against those who strike against them, but, on the other hand, once the enemies cease to attack, Muslims are then commanded to stop attacking.

One of the grievances enumerated in the American Declaration of Independence was that King George III "(...) has endeavoured to bring on the inhabitants of our frontiers the merciless Indian Savages whose known rule of warfare is an undistinguished destruction of all ages, sexes and conditions."

General Principles. "Certain fundamental principles provide basic guidance. For instance, the principles of distinction, proportionality, and necessity, all of which are part of customary international law, always apply to the use of armed force".[1]

The opposite of positive laws of war is customary laws of war,[1] many of which were explored at the Nuremberg War Trials. These laws define both the permissive rights of states as well as prohibitions on their conduct when dealing with irregular forces and non-signatories.

The Lieber Code, promulgated by the Union during the American Civil War, was critical in the development of the laws of land warfare.[7] Historian Geoffrey Best called the period from 1856 to 1909 the law of war’s “epoch of highest repute.”[8] The defining aspect of this period was the establishment, by states, of a positive legal or legislative foundation (i.e., written) superseding a regime based primarily on religion, chivalry, and customs.[9] It is during this “modern” era that the international conference became the forum for debate and agreement between states and the “multilateral treaty” served as the positive mechanism for codification.

In addition, the Nuremberg War Trial judgment on "The Law Relating to War Crimes and Crimes Against Humanity"[10] held, under the guidelines Nuremberg Principles, that treaties like the Hague Convention of 1907, having been widely accepted by "all civilised nations" for about half a century, were by then part of the customary laws of war and binding on all parties whether the party was a signatory to the specific treaty or not.

Military necessity is governed by several constraints: an attack or action must be intended to help in the military defeat of the enemy; it must be an attack on a military objective,[13] and the harm caused to civilians or civilian property must be proportional and not excessive in relation to the concrete and direct military advantage anticipated.[14]

Distinction is a principle under international humanitarian law governing the legal use of force in an armed conflict, whereby belligerents must distinguish between combatants and civilians.[a][15]

Proportionality is a principle under international humanitarian law governing the legal use of force in an armed conflict, whereby belligerents must make sure that the harm caused to civilians or civilian property is not excessive in relation to the concrete and direct military advantage anticipated by an attack on a military objective.[14]

To fulfill the purposes noted above, the laws of war place substantive limits on the lawful exercise of a belligerent’s power. Generally speaking, the laws require that belligerents refrain from employing violence that is not reasonably necessary for military purposes and that belligerents conduct hostilities with regard for the principles of humanity and chivalry.

However, because the laws of war are based on consensus, the content and interpretation of such laws are extensive, contested, and ever-changing.[16] The following are particular examples of some of the substance of the laws of war, as those laws are interpreted today.

Modern laws of war regarding conduct during war (jus in bello), such as the 1949 Geneva Conventions, provide that it is unlawful for belligerents to engage in combat without meeting certain requirements, such as wearing distinctive uniform or other distinctive signs visible at a distance, carrying weapons openly, and conducting operations in accordance with the laws and customs of war. Impersonating enemy combatants by wearing the enemy’s uniform is allowed, though fighting in that uniform is unlawful perfidy, as is the taking of hostages.

Combatants also must be commanded by a responsible officer. That is, a commander can be held liable in a court of law for the improper actions of his or her subordinates. There is an exception to this if the war came on so suddenly that there was no time to organize a resistance, e.g. as a result of a foreign occupation.[citation needed]

In either case, persons protected by the Red Cross/Crescent or white flag are expected to maintain neutrality, and may not engage in warlike acts. In fact, engaging in war activities under a protected symbol is itself a violation of the laws of war known as perfidy. Failure to follow these requirements can result in the loss of protected status and make the individual violating the requirements a lawful target.[citation needed]

The law of war is binding not only upon States as such but also upon individuals and, in particular, the members of their armed forces. Parties are bound by the laws of war to the extent that such compliance does not interfere with achieving legitimate military goals. For example, they are obliged to make every effort to avoid damaging people and property not involved in combat or the war effort, but they are not guilty of a war crime if a bomb mistakenly or incidentally hits a residential area.

By the same token, combatants that intentionally use protected people or property as human shields or camouflage are guilty of violations of the laws of war and are responsible for damage to those that should be protected.

During conflict, punishment for violating the laws of war may consist of a specific, deliberate and limited violation of the laws of war in reprisal.

After a conflict has ended, persons who have committed or ordered any breach of the laws of war, especially atrocities, may be held individually accountable for war crimes through process of law. Also, nations which signed the Geneva Conventions are required to search for, then try and punish, anyone who has committed or ordered certain "grave breaches" of the laws of war. (See GC III, Art. 129 and Art. 130.)

Combatants who break specific provisions of the laws of war are termed unlawful combatants. Unlawful combatants who have been captured may lose the status and protections that would otherwise be afforded to them as prisoners of war, but only after a "competent tribunal" has determined that they are not eligible for POW status (See, e.g., GC III Art 5.) At that point, an unlawful combatant may be interrogated, tried, imprisoned, and even executed for their violation of the laws of war pursuant to the domestic law of their captor, but they are still entitled to certain additional protections, including that they be "treated with humanity and, in case of trial, shall not be deprived of the rights of fair and regular trial." (GC IV Art 5.)

For example in 1976, foreign soldiers fighting for the National Liberation Front of Angola (FNLA) were captured by the People's Movement for the Liberation of Angola (MPLA) in the civil war that broke out when Angola gained independence from Portugal in 1975. In the Luanda Trial, after "a regularly constituted court" found them guilty of being unlawful mercenaries, three Britons and an American were shot by a firing squad on July 10, 1976. Nine others were imprisoned for terms of 16 to 30 years.

1874 Project of an International Declaration concerning the Laws and Customs of War (Brussels Declaration).[21] Signed in Brussels 27 August. This agreement never entered into force, but formed part of the basis for the codification of the laws of war at the 1899 Hague Peace Conference.[22][23]

1880 Manual of the Laws and Customs of War at Oxford. At its session in Geneva in 1874 the Institute of International Law appointed a committee to study the Brussels Declaration of the same year and to submit to the Institute its opinion and supplementary proposals on the subject. The work of the Institute led to the adoption of the Manual in 1880 and it went on to form part of the basis for the codification of the laws of war at the 1899 Hague Peace Conference.[23]

1899 Hague Conventions consisted of four main sections and three additional declarations (the final main section is for some reason identical to the first additional declaration):

I – Pacific Settlement of International Disputes

II – Laws and Customs of War on Land

III – Adaptation to Maritime Warfare of Principles of Geneva Convention of 1864

IV – Prohibiting Launching of Projectiles and Explosives from Balloons

Declaration I – On the Launching of Projectiles and Explosives from Balloons

Declaration II – On the Use of Projectiles the Object of Which is the Diffusion of Asphyxiating or Deleterious Gases

Declaration III – On the Use of Bullets Which Expand or Flatten Easily in the Human Body

1907 Hague Conventions had thirteen sections, of which twelve were ratified and entered into force, and two declarations:

I – The Pacific Settlement of International Disputes

II – The Limitation of Employment of Force for Recovery of Contract Debts

III – The Opening of Hostilities

IV – The Laws and Customs of War on Land

V – The Rights and Duties of Neutral Powers and Persons in Case of War on Land

VI – The Status of Enemy Merchant Ships at the Outbreak of Hostilities

VII – The Conversion of Merchant Ships into War – Ships

VIII – The Laying of Automatic Submarine Contact Mines

IX – Bombardment by Naval Forces in Time of War

X – Adaptation to Maritime War of the Principles of the Geneva Convention

XI – Certain Restrictions with Regard to the Exercise of the Right of Capture in Naval War

XII – The Creation of an International Prize Court [Not Ratified]*

XIII – The Rights and Duties of Neutral Powers in Naval War

Declaration I – extending Declaration II from the 1899 Conference to other types of aircraft

^Civilian in this instance means civilians who are non-combatants. Article 51.3 of Protocol I to the Geneva Conventions explains that "Civilians shall enjoy the protection afforded by this section, unless and for such time as they take a direct part in hostilities".

^Article 52 of Additional Protocol I to the Geneva Conventions provides a widely accepted definition of military objective: "In so far as objects are concerned, military objectives are limited to those objects which by their nature, location, purpose or use make an effective contribution to military action and whose total or partial destruction, capture or neutralization, in the circumstances ruling at the time, offers a definite military advantage" (Source: Moreno-Ocampo 2006, page 5, footnote 11).

^Jefferson D. Reynolds. "Collateral Damage on the 21st century battlefield: Enemy exploitation of the law of armed conflict, and the struggle for a moral high ground".Air Force Law ReviewVolume 56, 2005(PDF) Page 57/58 "if international law is not enforced, persistent violations can conceivably be adopted as customary practice, permitting conduct that was once prohibited"